You should be able to add chips as long as you have a pin available for chip-select. However, I haven't specifically tried multiple chips. I don't know if there is a practical limit to the #of chips one can tie onto the SPI pins. Each chip will add a certain amount of capacitive loading on the lines.

You might better look at the 23LC1024 chips = 128KB, plus they work at both 3.3 and 5V. The 23K256 only works at Vcc=3.3V, and you need level-shifters to connect to a 5V arduino.